Ceramic honeycomb filter and its production method

ABSTRACT

A ceramic honeycomb filter including a ceramic honeycomb structure having large numbers of flow paths partitioned by porous cell walls, and plugs disposed in the flow paths alternately on the exhaust gas inlet or outlet side, to remove particulate matter from an exhaust gas passing through the porous cell walls; the porous cell walls having porosity of 45-75%, the median pore diameter A (μm) of the cell walls measured by mercury porosimetry, and the median pore diameter B (μm) of the cell walls measured by a bubble point method meeting the formula of 35&lt;(A−B)/B×100≦70, and the maximum pore diameter of the cell walls measured by a bubble point method being 100 μm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 13/638,283filed Sep. 28, 2012, which is a National Stage of InternationalApplication No. PCT/JP2011/058132 filed Mar. 30, 2011 (claiming prioritybased on Japanese Patent Application No. 2010-085179 filed Apr. 1,2010), the contents of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to a ceramic honeycomb filter for removingparticulate matter from exhaust gases discharged from diesel engines,and its production method.

BACKGROUND OF THE INVENTION

An exhaust gas discharged from diesel engines contains particulatematter (PM) comprising as main components carbonaceous soot and solubleorganic fractions (SOFs) comprising high-boiling-point hydrocarboncomponents, which are likely to adversely affect humans and environmentwhen discharged into the air. Accordingly, ceramic honeycomb filtershave conventionally been attached to exhaust pipes of diesel engines forremoving PM. One example of ceramic honeycomb filters for capturing PMin the exhaust gas is shown in FIGS. 1 and 2. A ceramic honeycomb filter10 comprises a ceramic honeycomb structure comprising porous cell walls2 defining a large number of outlet-side-sealed flow paths 3 andinlet-side-sealed flow paths 4 and a peripheral wall 1, andupstream-side plugs 6 a and downstream-side plugs 6 c sealing theexhaust-gas-inlet-side end surface 8 and exhaust-gas-outlet-side endsurface 9 of the outlet-side-sealed flow paths 3 and theinlet-side-sealed flow paths 4 alternately in a checkerboard pattern.

As shown in FIG. 2, this ceramic honeycomb filter 10 is gripped bysupport members 14 and longitudinally sandwiched by support members 13a, 13 b in a metal container 12. The support members 14 are generallyformed by metal meshes and/or ceramic mats. The ceramic honeycomb filter10 mounted to a diesel engine receives mechanical vibration and shockfrom the engine, road surfaces, etc. via the support members 13 a, 13 band 14. Because such large ceramic honeycomb filters as having outerdiameters of more than 200 mm are subject to a large load by vibrationand shock, they are required to have high strength.

Among the characteristics required for ceramic honeycomb filters,PM-capturing efficiency, pressure loss, and a PM-capturable time period(a time period until pressure loss reaches a predetermined level fromthe start of capturing) are important. Particularly the capturingefficiency and the pressure loss are in a contradictory relation, highercapturing efficiency leading to larger pressure loss and thus a shorterPM-capturable time period. A low-pressure-loss design provides lowcapturing efficiency, despite a long PM-capturable time period. Tosatisfy all of these contradictory filter characteristics, investigationhas conventionally been conducted to provide technologies of controllingthe porosity, pore size distribution, etc. of the ceramic honeycombstructure.

JP 2003-534229 A discloses a ceramic structure having a cordierite phaseas a main component, and a thermal expansion coefficient of more than4×10⁻⁷/° C. and less than 13×10⁻⁷/° C. between 25° C. and 800° C., itspermeability and pore size distribution meeting the formula of2.108×(permeability)+18.511×(total pore volume)+0.1863×(percentage ofpores of 4-40 μm to total pore volume)>24.6.

JP 2007-525612 A discloses a filter for capturing diesel particulatematter, which has a median diameter d50 of less than 25 μm, and a poresize distribution and porosity meeting the relation of Pm≦3.75, whereinPm is expressed by Pm=10.2474 [1/((d50)² (% porosity/100))]+0.0366183(d90)−0.00040119 (d90)²+0.468815 (100/% porosity)²+0.0297715(d50)+1.61639 (d50−d10)/d50, wherein d10, d50 and d90 (d10<d50<d90)represents pore diameters at cumulative pore sizes (by volume) of 10%,50% and 90%, respectively.

The technologies described in JP 2003-534229 A and JP 2007-525612 Arestrict only pore structures (size and distribution) measured bymercury porosimetry to predetermined ranges, but they fail to designceramic honeycomb filters capable of efficiently capturing nano-sizedPM, which are considered to have particularly large influence on humans,with small pressure loss.

JP 2006-095352 A discloses a honeycomb filter having porosity of 45-70%,which has cell walls formed by a porous substrate having an average porediameter A (μm) measured by mercury porosimetry, and an average porediameter B (μm) measured by a bubble point method, an average porediameter difference ratio [(A−B)/B]×100 being 35% or less, the averagepore diameter B being 15-30 μm, and the maximum pore diameter measuredby a bubble point method being 150 μm or less.

JP 2006-095352 A describes that the average pore diameter A measured bymercury porosimetry is a value reflecting the average diameter of poreson cell wall surfaces, while the average pore diameter B measured by abubble point method is a value reflecting the minimum pore diameter inthe cell walls, that therefore, in the case of cell walls having a porestructure as shown in FIG. 4 (a), in which pores in the cell walls havesmall diameters, while those on cell wall surfaces have large diameters,the average pore diameter B measured by the bubble point method is muchsmaller than the average pore diameter A measured by mercuryporosimetry, and that on the other hand, in the case of cell wallshaving a pore structure as shown in FIG. 4 (b), in which pores in and onthe cell walls have the same diameters, and in the case of cell wallshaving a pore structure as shown in FIG. 4 (c), in which pores in thecell walls are larger than those on cell wall surfaces, the average porediameters A and B measured by mercury porosimetry and the bubble pointmethod are not substantially different.

JP 2006-095352 A describes that cell walls having the average porediameter difference ratio of 35% or less, namely small differencebetween the average pore diameter A measured by mercury porosimetry andthe average pore diameter B measured by the bubble point method, have astructure which comprises a smaller number of large pores on cell wallsurfaces than that of small pores in the cell walls [FIG. 4 (a)], aratio of diameters in the cell walls to those on cell wall surfacesbeing relatively small; namely, there are many pores having similardiameters in and on the cell walls [FIG. 4 (b)], and many smaller poreson cell wall surfaces than those in the cell walls [FIG. 4 (c)]. Namely,the honeycomb filter described in JP 2006-095352 A is constituted bycell walls having many pores as shown in FIGS. 4 (b) and 4 (c).

Because honeycomb filters shown in Examples of JP 2006-095352 A have themaximum pore diameter measured by the bubble point method in a range of129-145 μm, it is expected that pores in the cell walls have largerdiameters. Accordingly, the honeycomb filters have insufficientefficiency of capturing nano-sized PM having particularly largeinfluence on humans, despite small pressure loss.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a ceramichoneycomb filter having low pressure loss as well as improved efficiencyof capturing PM, particularly nano-sized PM, and its production method.

SUMMARY OF THE INVENTION

Thus, the ceramic honeycomb filter of the present invention comprises aceramic honeycomb structure having large numbers of flow pathspartitioned by porous cell walls, and plugs disposed in the flow pathsalternately on the exhaust gas inlet or outlet side, to removeparticulate matter from exhaust gas passing through the porous cellwalls,

the porous cell walls having porosity of 45-75%,

the median pore diameter A (μm) of the cell walls measured by mercuryporosimetry, and the median pore diameter B (μm) of the cell wallsmeasured by a bubble point method meeting the formula of35<(A−B)/B×100≦70, and

the maximum pore diameter of the cell walls measured by a bubble pointmethod being 100 μm or less.

The ceramic honeycomb filter of the present invention preferably has abulk density of 0.5 g/cm³ or less.

The ceramic honeycomb filter of the present invention preferably has athermal expansion coefficient of 13×10⁻⁷/° C. or less between 20° C. and800° C.

The porous cell walls preferably have permeability of 2×10⁻¹² m² to10×10⁻¹² m².

It is preferable that the porous cell walls have porosity of 55-70%,that the median pore diameter A of the cell walls measured by mercuryporosimetry is 25-35 μm, and that the volume of pores having diametersof 15-40 μm is 60-90% of the total pore volume.

The volume of pores having diameters of more than 50 μm is preferablymore than 10% and 23% or less of the total pore volume when the porouscell walls are measured by mercury porosimetry.

A main component of a crystal phase constituting the ceramic honeycombstructure is preferably cordierite.

The bulk filter density is preferably 0.4 g/cm³ or less, more preferably0.3 g/cm³ or less.

The thermal expansion coefficient of the ceramic honeycomb structurebetween 20° C. and 800° C. is preferably 10×10⁻⁷/° C. or less, morepreferably 8×10⁻⁷/° C. or less.

A honeycomb rod cut out of the ceramic honeycomb structure in parallelto the direction of the flow paths preferably has a bending strength of1 MPa or more when measured by a 4-point measurement method.

A honeycomb rod cut out of the ceramic honeycomb structure in parallelto the direction of the flow paths preferably has a Young's modulus of0.5 GPa or more.

The method of the present invention for producing a ceramic honeycombfilter comprises the steps of blending a starting material powdercomprising a cordierite-forming material comprising talc, silica, analumina source and kaolin, and a pore-forming material to prepare amoldable material, extruding the moldable material to form ahoneycomb-shaped molding, and plugging the predetermined flow paths ofthe honeycomb-shaped molding to form the ceramic honeycomb filter,

the silica having a median diameter of 15-60 μm,

the talc having a median diameter of 10-25 μm and a morphology index of0.77 or more,

the kaolin particles having a median diameter of 1-8 μm and a cleavageindex of 0.9 or more, the cleavage index being a value expressed byI₍₀₀₂₎/[I₍₂₀₀₎+I₍₀₂₀₎+I₍₀₀₂₎], wherein I₍₂₀₀₎, I₍₀₂₀₎ and I₍₀₀₂₎ are thepeak intensities of (200), (020) and (002) planes measured by X-raydiffraction,

the alumina source having a median diameter of 1-6 μm,

the pore-forming material having a median diameter of 30-70 μm, and

the cordierite-forming material being classified by passing through asieve having opening diameters of 250 μm or less.

In a curve showing the relation between a particle diameter and acumulative volume in the pore-forming material, a particle diameter d90at a cumulative volume corresponding to 90% of the total volume ispreferably 50-90 μm.

The alumina source preferably has a median diameter of 2-5 μm.

The silica preferably has a median diameter of 35-55 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view schematically showing one example of ceramichoneycomb filters.

FIG. 2 is a schematic cross-sectional view showing one example ofceramic honeycomb filters disposed in a metal container.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[1] Structure of Ceramic Honeycomb Filter

The ceramic honeycomb filter of the present invention comprises aceramic honeycomb structure having large numbers of flow pathspartitioned by porous cell walls, and plugs disposed in the flow pathsalternately on the exhaust gas inlet or outlet side, to removeparticulate matter from an exhaust gas passing through the porous cellwalls; the porous cell walls having porosity of 45-75%, the median porediameter A (μm) of the cell walls measured by mercury porosimetry, andthe median pore diameter B (μm) of the cell walls measured by a bubblepoint method meeting the formula of 35<(A−B)/B×100≦70, and the maximumpore diameter of the cell walls measured by a bubble point method being100 μm or less. This ceramic honeycomb filter has improved efficiency ofcapturing PM, particularly nano-sized PM, while keeping low pressureloss.

When the porous cell walls have porosity of less than 45%, it isimpossible to keep low pressure loss. On the other hand, when theporosity exceeds 75%, the porous cell walls do not have enough strengthto endure use. The porosity is preferably 50-73%, more preferably55-70%.

The formula of 35<(A−B)/B×100≦70 means that the median pore diameter Ais larger than the median pore diameter B, and that a ratio of themedian pore diameter A to the median pore diameter B is in a range of1.35<A/B≦1.7. The median pore diameter A measured by mercury porosimetryis a value reflecting the average diameter of pores on cell wallsurfaces, while the median pore diameter B measured by a bubble pointmethod is a value reflecting the minimum diameters of pores in the cellwalls. Accordingly, meeting the above formula means that the diametersof pores on cell wall surfaces are larger than those of pores in thecell walls in the ceramic honeycomb filter, and that their ratio isrelatively large, namely the diameters of pores decrease from thesurface to the inside with a large changing ratio. Therefore, theceramic honeycomb filter has good nano-sized PM-capturing efficiencywhile keeping low pressure loss. Further, the maximum pore diameter of100 μm or less measured by a bubble point method improves the efficiencyof capturing nano-sized PM, which is considered to have large influenceon humans. To achieve higher PM-capturing efficiency with lower pressureloss, the maximum pore diameter measured by a bubble point method ispreferably 30 μm or more.

The value of [(A−B)/B×100] of 35% or less, namely small differencebetween the diameters of pores on cell wall surfaces and those of poresin the cell walls, provides a low efficiency of capturing nano-sized PM.When the above value exceeds 70%, it is difficult to keep low pressureloss. The value of [(A−B)/B×100] is preferably 40-65%. When the maximumpore diameter measured by a bubble point method exceeds 100 μm, pores inthe cell walls have large diameters, resulting in insufficientefficiency of capturing PM, particularly nano-sized PM, which isconsidered to have large influence on humans. Also, the diameters ofpores on cell wall surfaces become relatively large, resulting in lowstrength.

The mercury porosimetry is a method of pressurizing a cell wall sampleimmersed in mercury in vacuum, and obtaining the relation betweenpressure when pressurizing and the volume of mercury forced into poresin the sample to determine the pore diameter distribution of the sample.In the mercury porosimetry, as pressure gradually increases, mercuryfirst enter larger surface pores and then smaller pores, finally fillingall pores. The total pore volume (total volume of entire pores) isdetermined from the amount of mercury forced into all pores, and a porediameter at which the cumulative volume of mercury forced into porescorresponds to 50% of the total pore volume is regarded as a median porediameter measured by mercury porosimetry.

The bubble point method is a method for determining pore diameters of acell wall sample by immersing the cell wall sample in a liquid having aknown surface tension to make the sample fully wet with the liquid,applying a gas pressure to one surface of the sample so that the gasflows through the cell wall sample, and determining the pore diametersby the amount of the flowing gas. As the gas pressure increases, theliquid contained in pores are forced out from another surface, so thatthe gas flows through the sample, and when the gas pressure gas furtherincreases, the flow rate of the gas increases. A pore diameterdistribution can be determined by measuring the pressure applied and theflow rate of the gas. A pore diameter at which the flow rate correspondsto 50% of a saturated flow rate is regarded as a median pore diametermeasured by the bubble point method. A pore diameter corresponding tothe smallest pressure permitting a gas flow is the maximum pore diameterin the sample.

The bulk density of the ceramic honeycomb filter is preferably 0.5 g/cm³or less. The bulk filter density is a quotient obtained by dividing themass of the honeycomb filter by the total volume of the honeycombfilter. With the bulk filter density of 0.5 g/cm³ or less, an exhaustgas flows through the honeycomb filter with small resistance, resultingin low pressure loss. On the other hand, when the bulk filter densityexceeds 0.5 g/cm³, low pressure loss is unlikely kept. The bulk filterdensity is preferably 0.4 g/cm³ or less, more preferably 0.3 g/cm³ orless. The bulk filter density is preferably 0.2 g/cm³ or more to haveenough strength.

The ceramic honeycomb filter preferably has a thermal expansioncoefficient of 13×10⁻⁷ or less between 20° C. and 800° C. With thethermal expansion coefficient of 13×10⁻⁷ or less, the ceramic honeycombfilter can maintain heat shock resistance with practically durablestrength when used as a filter for removing particulate matter from anexhaust gas of diesel engines. The thermal expansion coefficient between20° C. and 800° C. is preferably 10×10⁻⁷ or less, more preferably 3×10⁻⁷to 8×10⁻⁷.

To keep low pressure loss, the permeability of the porous cell walls ispreferably 2×10⁻¹² m² to 10×10⁻¹² m². The permeability of less than2×10⁻¹² m² is likely to increase the pressure loss, and the permeabilityof more than 10×10⁻¹² m² is likely to deteriorate the PM-capturingperformance. The permeability is more preferably more than 3×10⁻¹² m²and 8×10⁻¹² m² or less.

The median pore diameter A of the porous cell walls measured by mercuryporosimetry is preferably 25-35 μm. The median pore diameter A of lessthan 25 μm is likely to lower the pressure loss characteristics, and themedian pore diameter A exceeding 35 μm results in as low strength aspractically unacceptable. The average pore diameter A is preferably26-34 μm, more preferably 27-33 μm.

The volume of pores having diameters of 15-40 μm in the porous cellwalls measured by mercury porosimetry is preferably 60-90% of the totalpore volume. When the volume of pores having diameters of 15-40 μm isless than 60% of the total pore volume, there are many fine pores havingdiameters of less than 15 μm, which adversely affect the pressure losscharacteristics, and many large pores having diameters of more than 40μm, which are likely to lower the strength. On the other hand, when thevolume of pores having diameters of 15-40 μm exceeds 90% of the totalpore volume, fine pores having diameters of less than 15 μm and largepores having diameters of more than 40 μm are in small percentages,likely failing to secure the communications of pores to have lowpressure loss. The total volume of pores having diameters of 15-40 μm ispreferably 65-85%, more preferably 65-80%.

The volume of pores having diameters of more than 50 μm measured bymercury porosimetry is preferably more than 10% and 23% or less of thetotal pore volume. When the volume of pores having diameters of morethan 50 μm is 10% or less, the pressure loss characteristics are likelydeteriorated. When it is more than 25%, the percentage of large pores islikely large, resulting in low strength. The total volume of poreshaving diameters exceeding 50 μm is preferably 11-22%, more preferably12-21%.

The ceramic honeycomb structure preferably comprises cordierite as amain component of its crystal phase, with 3-6% by mass of spinel and1-8% by mass of cristobalite. With such crystal composition, sizechanges (expansion) by sintering can be suppressed, providing thesintered ceramic honeycomb structure with small size changes. The morepreferred crystal composition contains 4-5% by mass of spinel, and 2-7%by mass of cristobalite. The crystal phase may contain mullite,corundum, tridymite, etc., in addition to cordierite, spinel andcristobalite.

A honeycomb rod cut out of the ceramic honeycomb structure in parallelto the direction of flow paths preferably has a bending strength of 1MPa or more when measured by a 4-point measurement method. With suchbending strength, ceramic honeycomb filters having enough strength tobear use can be obtained. The bending strength is preferably 2 MPa ormore.

A honeycomb rod cut out of the ceramic honeycomb structure in parallelto the direction of flow paths preferably has a Young's modulus of 0.5GPa or more. With such Young's modulus, ceramic honeycomb filters havingenough strength to bear use can be obtained. With the Young's modulus ofless than 0.5 GPa, the ceramic honeycomb filters are likely deformed andbroken by vibration or shock during use. The Young's modulus ispreferably 1 GPa or more.

[2] Production Method of Ceramic Honeycomb Filter

The production method of the ceramic honeycomb filter of the presentinvention comprises the steps of blending a starting material powdercomprising a cordierite-forming material comprising talc, silica, analumina source and kaolin, and a pore-forming material to prepare amoldable material, extruding the moldable material to form ahoneycomb-shaped molding, and plugging the predetermined flow paths ofthe honeycomb-shaped molding to form the ceramic honeycomb filter; thesilica having a median diameter of 15-60 μm, the talc having a mediandiameter of 10-25 μm and a morphology index of 0.77 or more, the kaolinparticles having a median diameter of 1-8 μm and a cleavage index of 0.9or more, the cleavage index being a value expressed byI₍₀₀₂₎/[I₍₂₀₀₎+I₍₀₂₀₎+I₍₀₀₂₎], wherein I₍₂₀₀₎, I₍₀₂₀₎, and I₍₀₀₂₎ arethe peak intensities of (200), (020) and (002) planes measured by X-raydiffraction, the alumina source having a median diameter of 1-6 μm, thepore-forming material having a median diameter of 30-70 μm, and thecordierite-forming material being classified by passing through a sievehaving opening diameters of 250 μm or less, to limit the percentage offine pores deteriorating pressure loss characteristics and large poreslowering strength, thereby increasing the percentage of pores necessaryfor keeping low pressure loss.

This production method provides the ceramic honeycomb filter of thepresent invention, which comprises porous cell walls (i) having porosityof 45-75%, the median pore diameter A (μm) measured by mercuryporosimetry and the median pore diameter B (μm) measured by a bubblepoint method meeting the formula of 35<(A−B)/B×100≦70, and the maximumpore diameter measured by a bubble point method being 100 μm or less,and has (ii) a bulk density of 0.5 g/cm³ or less, (iii) a thermalexpansion coefficient of 13×10⁷/° C. or less between 20° C. and 800° C.,and (iv) permeability of 2×10⁻¹² m² to 10×10⁻¹² m².

The ceramic material is preferably a cordierite-forming material. Thecordierite-forming material is obtained by mixing silica source powder,alumina source powder and magnesia source powder, such that theresultant main crystal is cordierite having a chemical compositioncomprising as main components 42-56% by mass of SiO₂, 30-45% by mass ofAl₂O₃ and 12-16% by mass of MgO. Pores formed in the ceramic comprisingcordierite as a main crystal are mainly constituted by pores formed bysintering silica, and pores formed by burning off the pore-formingmaterial.

(a) Silica Particles

It is known that silica is more stable than other materials up to hightemperatures, and melt-diffused at 1300° C. or higher to form pores. Thepresence of 10-25% by mass of silica in the cordierite-forming materialprovides a desired amount of pores. With more than 25% by mass of silicacontained, the amounts of kaolin and/or talc, other silica sources,should be reduced to keep cordierite as a main crystal, decreasing theeffect of kaolin of reducing thermal expansion (such effect is obtainedby the orientation of kaolin during extrusion-molding), and thusresulting in low heat shock resistance. On the other hand, when silicais less than 10% by mass, the number of pores open on cell wall surfacesis small, likely failing to keep low pressure loss when PM is capturedand accumulated. The amount of silica is preferably 12-22% by mass.

The silica particles used have a median diameter of 15-60 μm. When themedian diameter of silica particles is less than 1.5 μm, many fine poresdeteriorating the pressure loss characteristics are formed, and when itis more than 60 μm, many large pores lowering the strength are formed.The median diameter of silica particles is preferably 35-55 μm.

Though the silica particles may be crystalline or amorphous, they arepreferably amorphous to control the particle size distribution.Amorphous silica can be obtained by melting high-purity, natural silicastone at high temperatures and pulverizing the resultant ingot. Thoughsilica particles may contain Na₂O, K₂O and CaO as impurities, the totalamount of the above impurities is preferably 0.1% or less to avoid alarge thermal expansion coefficient.

The silica particles preferably have sphericity of 0.5 or more. When thesphericity of silica particles is less than 0.5, many fine poresdeteriorating the pressure loss characteristics and many large poreslowering the strength are formed. The sphericity of silica particles ispreferably 0.6 or more, more preferably 0.7 or more. The sphericity of asilica particle is a value obtained by dividing a projected area of thesilica particle by the area of a circle, whose diameter is the longeststraight line passing a center of gravity of the silica particle andconnecting two points on a periphery of the particle, which isdetermined from an electron photomicrograph by an image analyzer.

High-sphericity silica particles can be obtained by spraying finelypulverized, high-purity, natural silica stone into a high-temperatureflame. Spraying into a high-temperature flame causes the melting andspheroidization of silica particles simultaneously, providinghigh-sphericity, amorphous silica. The spherical silica particles arepreferably adjusted with respect to particle size by classification,etc.

(b) Talc

The talc has a median diameter of 10-25 μm. As a magnesia component,35-45% by mass of talc is preferably added. The talc may contain Fe₂O₃,CaO, Na₂O, K₂O, etc. as impurities. the amount of Fe₂O₃ in the talc ispreferably 0.5-2.5% by mass to obtain the desired particle sizedistribution, and the total amount of Na₂O, K₂O and CaO is preferably0.5% or less by mass to lower the thermal expansion coefficient.

To reduce the thermal expansion coefficient of the ceramic honeycombstructure whose main crystal phase is cordierite, talc particles arepreferably in a plate-like shape. The morphology index, which is ameasure of the platyness of talc particles, is preferably 0.77 or more,more preferably 0.8 or more, most preferably 0.83 or more. The abovemorphology index is, as described in U.S. Pat. No. 5,141,686, determinedby the following formula:Morphology index=Ix/(Ix+2Iy),wherein Ix and Iy respectively represent the diffraction intensities of(004) and (020) planes of talc, which are obtained by the X-raydiffraction measurement of planar talc particles. The larger themorphology index, the more platy the talc particles.

The amount of talc added to the cordierite-forming material ispreferably 40-43% by mass to form cordierite as a main crystal.

(c) Kaolin

Kaolin particles used have a median diameter of 1-8 μm. The ceramic cellwalls whose main crystal is cordierite have pores formed by firingmainly silica particles and pores formed by burning off the pore-formingmaterial in the sintering process. Because kaolin particles having amedian diameter of 1-8 μm smaller than those of silica and thepore-forming material form pores between pores formed by the silicaparticles and the pore-forming material, making these pores communicableeach other, a pore structure in which pores in the cell walls have smalldiameters with improved communicability, and pores on cell wall surfaceshave larger diameters than those of pores in the cell walls. As aresult, in the pore structure, the median pore diameter A (μm) of cellwalls measured by mercury porosimetry, and the median pore diameter B(μm) of cell walls measured by a bubble point method meet the conditionof 35<(A−B)/B×100≦70, and the maximum pore diameter of the cell wallsmeasured by a bubble point method is 100 μm or less. The median diameterof kaolin particles is preferably 2-6 μm.

When kaolin particles are oriented such that their c-axes areperpendicular to the longitudinal direction of the extrusion-moldedhoneycomb structure, the c-axes of cordierite crystals are in parallelto the longitudinal direction of the honeycomb structure, providing thehoneycomb structure with a small thermal expansion coefficient. Theshape of kaolin particles has large influence on the orientation ofkaolin particles. The cleavage index of kaolin particles, which is anindex indicating the shape of kaolin particles quantitatively, ispreferably 0.9 or more, more preferably 0.93 or more. The cleavage indexof kaolin particles can be determined by measuring the X-ray diffractionof press-molded kaolin particles to obtain the X-ray diffraction peakintensities I₍₂₀₀₎, I₍₀₂₀₎ and I₍₀₀₂₎ of (200), (020) and (002) planesof kaolin particles, and using the following formula:Cleavage index=I ₍₀₀₂₎ /[I ₍₂₀₀₎ +I ₍₀₂₀₎ +I ₍₀₀₂₎].The larger the cleavage index, the more the kaolin particles areoriented.

The cordierite-forming material preferably contains 1-15% by mass ofkaolin particles. More than 15% by mass of kaolin particles likelyincrease fine pores having diameters of less than 5 μm in the ceramichoneycomb structure, thereby deteriorating the pressure losscharacteristics, and less than 1% by mass of kaolin particles providethe ceramic honeycomb structure with a large thermal expansioncoefficient. The amount of the kaolin powder is more preferably 4-8% bymass.

(d) Alumina

The alumina source used has a median diameter of 1-6 μm. The aluminasource reduces the thermal expansion coefficient, and with a smallermedian diameter than those of silica particles and the pore-formingmaterial like the kaolin particles, it acts to make pores formed bysintering silica particles and pores formed by burning off thepore-forming material communicable each other. The alumina sourcepreferably has a median diameter of 2-5 μm.

The alumina source is preferably aluminum oxide and/or aluminumhydroxide because of little impurities. When aluminum hydroxide is used,its amount in the cordierite-forming material is preferably 6-42% bymass, more preferably 6-15% by mass, most preferably 8-12% by mass. Whenaluminum oxide is used, its amount in the cordierite-forming material ispreferably 30% or less by mass, more preferably 12-25% by mass, mostpreferably 20-24% by mass. The total amount of Na₂O, K₂O and CaO,impurities in aluminum oxide and aluminum hydroxide, is preferably 0.5%or less by mass, more preferably 0.3% or less by mass, most preferably0.1% or less by mass.

(e) Pore-Forming Material

The pore-forming material is burned off before cordierite is synthesizedin a cordierite-sintering process, forming pores. The pore-formingmaterial used has a median diameter of 30-70 μm. When the mediandiameter is less than 30 μm, a few pores having relatively largediameters are formed, failing to keep low pressure loss. When the mediandiameter exceeds 70 μm, too large pores are formed, failing to achievesufficient strength. The median diameter of the pore-forming material ispreferably 40-60 μm.

In a curve showing the relation between the particle diameters of thepore-forming material particles and their cumulative volume (theaccumulated volume of particles having diameters equal to or less than aparticular diameter), it is preferable that the particle diameter d9corresponding to a cumulative volume of 90% is 50-90 μm. When theparticle diameter d90 is less than 50 μm, cell wall surfaces have manypores having smaller diameters than those of pores in the cell walls,likely resulting in the deterioration of pressure loss characteristics.The d9 exceeding 90 μm means a large maximum pore diameter measured bythe bubble point method, likely resulting in a low efficiency ofcapturing nano-sized PM. The particle diameter d90 is preferably 60-80μm. The diameters of pore-forming material particles can be measured,for example, by a particle size distribution meter (Microtrack MT3000available from Nikkiso Co., Ltd.).

Usable for the pore-forming material are flour, graphite, starch powder,solid or hollow resins (polymethylmethacrylate, polybutylmethacrylate,polyacrylate, polystyrene, polyethylene, polyethylene terephthalate,methylmethacrylate-acrylonitrile copolymers, etc.), etc. Preferableamong them are hollow resin particles, particularly hollow particles ofmethylmethacrylate-acrylonitrile copolymers. The hollow resin particlespreferably have shells as thick as 0.1-2 μm and contain a gas such ashydrocarbons, etc. They preferably contain 70-95% of moisture. With themoisture contained, the resin particles have such improved slidabilitythat they are not easily broken in their mixing, blending and molding.

The amount of the pore-forming material added is preferably set in sucha range as to provide low pressure loss while securing high strength,depending on its type. When hollow resin particles are used as thepore-forming material, their amount is preferably 1-15%. With less than1%, a small amount of pores are formed by the pore-forming material,likely failing to keep low pressure loss. With more than 15%, too manypores are formed, likely failing to have sufficient strength. The amountof the pore-forming material added is more preferably more than 6% and15% or less, most preferably 6.5-13%. When the pore-forming material isflour, graphite, starch powder, etc., its amount is preferably in arange of 5-70%.

(f) Classification of Cordierite-Forming Material

The cordierite-forming material comprising silica particles, talcparticles, kaolin particles, alumina particles, etc., is classified bypassing through a sieve with openings of 250 μm or less. Coarseparticles are removed by the sieve from the cordierite-forming material,preventing pores on and in the cell walls from becoming large. The sieveopenings are preferably 220 μm or less.

(g) Production Method

A moldable material for the extrusion molding of a ceramic honeycombfilter is produced by mixing the cordierite-forming material comprisingsilica particles, talc particles, kaolin particles, alumina particles,etc. with the pore-forming material, a binder, etc., without usingpulverization media like a Henschel mixer, etc., and blended with waterwithout excess shearing like a kneader, etc. By mixing without usingpulverization media, silica particles (particularly amorphous silicaparticles) and the pore-forming material are not broken in the mixingstep, so that the extruded molding contains silica particles and thepore-forming material having desired particle size distributions andparticle shapes without deformation, resulting in a ceramic honeycombfilter meeting both requirements of pressure loss characteristics andPM-capturing efficiency. Particularly when high-sphericity silica andhollow resin particles as the pore-forming material are used, the mixingmethod is effective. When pulverization media are used like a ball mill,etc. in the mixing step, silica particles (particularly high-sphericitysilica particles), and hollow resin particles as the pore-formingmaterial are broken in the mixing step, so that their shapes andparticle sizes are changed, failing to obtain a desired pore structure.

The ceramic honeycomb structure is produced by extruding the plasticizedmoldable material through a die by a known method to faun a honeycombmolding, drying it, machining its end surfaces and peripheral surface,etc., if necessary, and sintering it. The sintering is conducted in acontinuous or batch furnace, with heating and cooling speeds adjusted.When the ceramic material is a cordierite-forming material, it is keptat 1350-1450° C. for 1-50 hours to cause main cordierite crystalssufficiently grow, and then cooled to room temperature. Particularlywhen a large ceramic honeycomb structure of 150 mm or more in outerdiameter and 150 mm or more in length is produced, the heating speed ispreferably 0.2-10° C./hour in a temperature range of decomposing thebinder (for example, between 150° C. and 350° C.), and 5-20° C./hour ina temperature range of causing a cordierite-forming reaction (forexample, between 1150° C. and 1400° C.), to avoid the cracking ofmoldings in the sintering step. The cooling is preferably conducted at aspeed of 20-40° C./hour particularly in a range of 1400-1300° C.

The resultant honeycomb ceramic structure is plugged at desired flowpath ends or inside desired flow paths by a known method, to form aceramic honeycomb filter. The plugging may be conducted beforesintering.

The present invention will be explained in further detail by Examplesbelow, without intention of restricting the present invention thereto.

Starting material powders of silica, kaolin, talc, alumina and aluminumhydroxide having the characteristics (median diameters, impurities,etc.) shown in Tables 1-4 are classified by passing through a sieve withopening diameters of 212 μm, and mixed by the formulations shown inTable 6, to obtain cordierite-forming material powders having a chemicalcomposition comprising 51% by mass of SiO₂, 35% by mass of Al₂O₃ and 14%by mass of MgO. Each of the cordierite-forming material powders wasmixed with each pore-forming material shown in Table 5 in an amountshown in Table 6, and methylcellulose, and blended with water to preparea ceramic moldable material composed of the cordierite-forming material.

The median diameters and particle size distributions of silica, kaolin,talc, alumina, aluminum hydroxide, and the pore-forming materials weremeasured by a particle size distribution meter (Microtrack MT3000). Thesphericity of each silica particle was determined from an electronphotomicrograph of the particle by an image analyzer by the formula ofA1/A2, wherein A1 was a projected area of the silica particle, and A2was an area of a circle having a diameter corresponding to the longeststraight line passing a center of gravity of the silica particle andconnecting two points on a periphery of the silica particle, and theresultant sphericity values of 20 silica particles were averaged toobtain the sphericity of silica particles.

The resultant moldable material was extruded to form a honeycombstructure molding, dried, machined to remove its peripheral portions,and sintered in a furnace for 200 hours by a schedule comprisingtemperature elevation at an average speed of 10° C./hour from roomtemperature to 150° C., at an average speed of 2° C./hour from 150° C.to 350° C., at an average speed of 20° C./hour from 350° C. to 1150° C.,and at an average speed of 15° C./hour from 1150° C. to 1400° C.,keeping the highest temperature of 1410° C. for 24 hours, and cooling atan average speed of 30° C./hour from 1400° C. to 1300° C., and at anaverage speed of 80° C./hour from 1300° C. to 100° C.

Peripheries of the sintered ceramic honeycombs were coated with a skinmaterial comprising amorphous silica and colloidal silica, and dried toobtain the ceramic honeycomb structures of Examples 1-16 and ComparativeExamples 1-12 each having an outer diameter of 266.7 mm, a length of304.8 mm, a cell wall thickness of 300 μm, and a cell density of 260cells/inch².

Flow path ends of each ceramic honeycomb structure were alternatelyplugged with a cordierite-forming, plugging material slurry, which wasdried and sintered to produce each cordierite-type ceramic honeycombfilter of Examples 1-16 and Comparative Examples 1-12. The sinteredplugs were as long as 5-10 mm.

The resultant ceramic honeycomb filters of Examples 1-16 and ComparativeExamples 1-12 were measured with respect to a pore distribution bymercury porosimetry, pore diameters by a bubble point method, a thermalexpansion coefficient, the amount of crystals, a bulk density,permeability, pressure loss when 2 g/liter of soot was captured,capturing efficiency, bending strength and a Young's modulus. Theresults are shown in Table 7.

The bubble point method and the mercury porosimetry were conducted ontest pieces cut out of the honeycomb filters. Measured on cell walls bythe mercury porosimetry were a total pore volume, porosity, a medianpore diameter A, a ratio of the volume of pores having diameters of15-40 μm to the total pore volume, and a ratio of the volume of poreshaving diameters of more than 50 μm to the total pore volume, andmeasured on cell walls by the bubble point method were a median porediameter B, and the maximum pore diameter.

The mercury porosimetry was conducted by putting a test piece (10 mm×10mm×10 mm) cut out of the ceramic honeycomb filter in a measurement cellof Autopore III available from Micromeritics, evacuating the cell,introducing mercury into the cell under pressure, and determining therelation between the pressure and the volume of mercury introduced intopores in the test piece. Determined from the relation between thepressure and the volume of mercury was the relation between the porediameter and the cumulative pore volume. With mercury pressure of 0.5psi (0.35×10⁻³ kg/mm²), the pore diameter was calculated from thepressure, using a contact angle of 130° and a surface tension of 484dyne/cm. The porosity was calculated from the measured total porevolume, using 2.52 g/cm³ as the true density of cordierite.

The measurement of pores by the bubble point method was conducted on atest piece cut out of the ceramic honeycomb filter, which was containedin PermPorometer CFP1100AEX available from Porous Materials, Inc., witha perfluoropolyester (trade name “Galwick”) dropped thereonto.

The measurement of the thermal expansion coefficient (CTE) between 20°C. and 800° C. was conducted on a test piece cut out of the honeycombfilter.

The amounts of crystals, cordierite, spinel and cristobalite, weredetermined from main peak intensities of the crystals obtained by X-raydiffraction measurement. With the X-ray diffraction pattern of a powdersample of the ceramic honeycomb filter obtained in a range of 2θ=8-40°using an X-ray diffractometer (Cu-Kα) available from Rigaku Corporation,the main peak intensities of crystals (the highest diffraction peakintensities of the crystals in an X-ray diffraction pattern) weredetermined from the diffraction intensity I_(cordierite (102)) of the(102) plane of cordierite, the diffraction intensity I_(spinel (220)) ofthe (220) plane of spinet, and the diffraction intensityI_(cristobalite (200)) of the (200) plane of cristobalite in the X-raydiffraction pattern, with their values converted. The conversion of themeasured intensities to the main peak intensities [X-ray diffractionintensities of the (500) plane of cordierite, the (311) plane of spinet,and the (101) plane of cristobalite] was conducted by the followingformulae:X-ray diffraction intensity of cordierite crystal=(I_(cordierite (102))/50)×100,X-ray diffraction intensity of spinel crystal=(I_(spinel (220))/40)×100, andX-ray diffraction intensity of cristobalite crystal =(I_(cristobalite (200))/13)×100,using an intensity ratio to the main peak intensity of each crystal, 50%for the (102) plane of cordierite, 40% for the (220) plane of spinet,and 13% for the (200) plane of cristobalite, which are described inJCPDS cards. Such conversion avoids, for example, the problem that exactintensities cannot be determined because crystals' main peaks areoverlapping, enabling the comparison of the amounts of crystals withhigher precision.

The amount of each crystal (cordierite, spinel, and cristobalite) wasdetermined by dividing the main peak intensity of each crystal by theirsum. For example, the amount of spinel crystal was determined by theformula:(I _(spinel (220))/40)×100/[(I _(cordierite (102))/50)×100+(I_(spinel (220))/40)×100+(I _(cristobalite (200))/13)×100].

The bulk filter density was determined by dividing the mass of thehoneycomb filter by the volume of the honeycomb filter.

The permeability was the maximum permeability measured by Perm AutomatedPorometer (registered trademark) Ver. 6.0 available from PorousMaterials, Inc., with the flow rate of air increasing from 30 cc/sec to400 cc/sec. Evaluation in Table 7 was as follows:

-   -   Excellent: When the permeability was more than 3×10⁻¹² m² and        8×10⁻¹² m² or less,    -   Good: When the permeability was 2×10⁻¹² m² to 3×10⁻¹² m², or        more than 8×10⁻¹² m² and 10×10⁻¹² m² or less, and    -   Poor: When the permeability was less than 2×10⁻¹² m² or more        than 10×10⁻¹² m².

The pressure loss when 2 g/liter of soot was captured (soot-capturingpressure loss) was measured on a ceramic honeycomb filter fixed to apressure loss test stand, to which carbon powder (soot) having anaverage particle size of 0.042 μm was supplied at a rate of 3 g/hourtogether with air in a flow rate of 10 Nm³/min, and expressed bypressure difference between the inlet side and the outlet side (pressureloss) when the amount of soot accumulated per 1 liter of the filterreached 2 g. The soot-capturing pressure loss was evaluated as follows:

-   -   Excellent When the pressure loss was 1.2 kPa or less,    -   Good When the pressure loss was more than 1.2 kPa and 1.5 kPa or        less,    -   Poor When the pressure loss was more than 1.5 kPa.

Supplying carbon powder having an average particle size of 0.042 μm at arate of 3 g/hour together with air at a flow rate of 10 Nm³/min to aceramic honeycomb filter fixed to a pressure loss test stand, thenumbers of carbon particles flowing into and out of the honeycomb filterper 1 minute were counted by a scanning mobility particle sizer (SMPS,Model 3936 available from TIS), to determine the capturing efficiency bythe formula of (N_(in)−N_(out))/N_(in), wherein N_(in) represents thenumber of carbon particles flowing into the honeycomb filter, andN_(out) represents the number of carbon particles flowing out of thehoneycomb filter, both for 1 minute between 20 minutes and 21 minutesfrom the start of supply. The capturing efficiency was evaluated by thefollowing standards:

-   -   Excellent: When the value of the above formula was 98% or more,    -   Good: When the value of the above formula was 95% or more and        less than 98%, and    -   Poor: When the value of the above formula was less than 95%.

The measurement of the bending strength and the Young's modulus wasconducted on a honeycomb rod of 100 mm in length, 12 mm in thickness inparallel to the flow path direction, and 25 mm in width, which was cutout of the ceramic honeycomb filter in the flow path direction, by a4-point bending test method with a distance of 80 mm between lowerfulcrums and with a distance of 40 mm between upper fulcrums.

TABLE 1 Median Starting Diameter Impurities (%) Material (μm) SphericityCaO Na₂O K₂O Silica A 45 0.70 0.001 0.0021 0.0024 Silica B 58 0.70 0.0010.0021 0.0024 Silica C 15 0.70 0.001 0.0021 0.0024 Silica D 45 0.900.001 0.0017 0.0021 Silica E 45 0.50 0.001 0.0017 0.0021 Silica F 700.70 0.001 0.0024 0.0023 Silica G 10 0.70 0.001 0.0024 0.0023 Silica H45 0.40 0.001 0.0024 0.0025

TABLE 2 Median Starting Diameter Cleavage Impurities (%) Material (μm)Index CaO Na₂O K₂O Kaolin A 4.5 0.95 0.19 0.03 0.09 Kaolin B 7.5 0.950.20 0.03 0.08 Kaolin C 1.5 0.95 0.20 0.03 0.08 Kaolin D 4.5 0.90 0.190.03 0.09 Kaolin E 10.0 0.95 0.19 0.03 0.09 Kaolin F 0.1 0.95 0.19 0.030.09 Kaolin G 4.5 0.70 0.19 0.03 0.09

TABLE 3 Median Starting Diameter Morphology Impurities (%) Material (μm)Index CaO Na₂O K₂O Fe₂O₃ Talc A 14.0 0.84 0.47 0.001 0.001 1.1 Talc B25.0 0.83 0.48 0.001 0.001 1.1 Talc C 10.0 0.83 0.48 0.001 0.001 1.1Talc D 14.0 0.77 0.49 0.001 0.001 1.1 Talc E 30.0 0.84 0.49 0.001 0.0011.1 Talc F 5.0 0.84 0.49 0.001 0.001 1.0 Talc G 14.0 0.60 0.49 0.0010.001 1.0

TABLE 4 Median Starting Diameter Impurities (%) Material (μm) CaO Na₂OK₂O Alumina A 4.0 0.001 0.28 0.001 Alumina B 6.0 0.001 0.30 0.001Alumina C 1.5 0.001 0.30 0.001 Alumina D 8.0 0.001 0.30 0.001 Alumina E0.2 0.001 0.30 0.001 Aluminum 1.8 0.001 0.04 0.001 Hydroxide A Aluminum7.0 0.001 0.04 0.001 Hydroxide B

TABLE 5 Median Starting Diameter d90 Material Material (μm) (μm)Pore-Forming Foamed Resin* 48.0 69.0 Material A Pore-Forming Graphite70.0 90.0 Material B Pore-Forming Flour 30.0 50.0 Material CPore-Forming Graphite 90.0 115.0 Material D Note: *Having a shellthickness of 1 μm with the water content of 90%.

TABLE 6 Silica Kaolin Talc Amount Amount Amount No. Type (%) Type (%)Type (%) Example 1 A 18.2 A 6.0 A 41.2 Example 2 B 18.2 A 6.0 A 41.2Example 3 C 18.2 A 6.0 A 41.2 Example 4 D 18.2 A 6.0 A 41.2 Example 5 E18.1 A 6.0 A 41.3 Comp. Ex. 1 F 18.1 A 6.0 A 41.3 Comp. Ex. 2 G 18.1 A6.0 A 41.3 Example 6 H 18.1 A 6.0 A 41.3 Example 7 A 18.1 B 6.0 A 41.3Example 8 A 18.1 C 6.0 A 41.3 Example 9 A 18.2 D 6.0 A 41.2 Comp. Ex. 3A 18.2 E 6.0 A 41.2 Comp. Ex. 4 A 18.2 F 6.0 A 41.2 Comp. Ex. 5 A 18.2 G6.0 A 41.2 Example 10 A 18.2 A 6.0 B 41.2 Example 11 A 18.2 A 6.0 C 41.2Example 12 A 18.2 A 6.0 D 41.2 Comp. Ex. 6 A 18.2 A 6.0 E 41.2 Comp. Ex.7 A 18.2 A 6.0 F 41.2 Comp. Ex. 8 A 18.2 A 6.0 G 41.2 Example 13 A 18.2A 6.0 A 41.2 Example 14 A 18.2 A 6.0 A 41.2 Comp. Ex. 9 A 18.2 A 6.0 A41.2 Comp. Ex. 10 A 18.2 A 6.0 A 41.2 Comp. Ex. 11 A 18.2 A 6.0 A 41.2Example 15 A 18.2 A 6.0 A 41.2 Example 16 A 18.2 A 6.0 A 41.2 Comp. Ex.12 A 18.2 A 6.0 A 41.2 Pore- Aluminum Forming Alumina Hydroxide MaterialAmount Amount Amount No. Type (%) Type (%) Type (%) Example 1 A 23.3 A11.3 A 8.0 Example 2 A 23.3 A 11.3 A 8.0 Example 3 A 23.3 A 11.3 A 8.0Example 4 A 23.3 A 11.3 A 8.0 Example 5 A 23.3 A 11.3 A 8.0 Comp. Ex. 1A 23.3 A 11.3 A 8.0 Comp. Ex. 2 A 23.3 A 11.3 A 8.0 Example 6 A 23.3 A11.3 A 8.0 Example 7 A 23.3 A 11.3 A 8.0 Example 8 A 23.3 A 11.3 A 8.0Example 9 A 23.3 A 11.3 A 8.0 Comp. Ex. 3 A 23.3 A 11.3 A 8.0 Comp. Ex.4 A 23.3 A 11.3 A 8.0 Comp. Ex. 5 A 23.3 A 11.3 A 8.0 Example 10 A 23.3A 11.3 A 8.0 Example 11 A 23.3 A 11.3 A 8.0 Example 12 A 23.3 A 11.3 A8.0 Comp. Ex. 6 A 23.3 A 11.3 A 8.0 Comp. Ex. 7 A 23.3 A 11.3 A 8.0Comp. Ex. 8 A 23.3 A 11.3 A 8.0 Example 13 B 23.3 A 11.3 A 8.0 Example14 C 23.3 A 11.3 A 8.0 Comp. Ex. 9 D 23.3 A 11.3 A 8.0 Comp. Ex. 10 E23.3 A 11.3 A 8.0 Comp. Ex. 11 A 23.3 B 11.3 A 8.0 Example 15 A 23.3 A11.3 B 25.0 Example 16 A 23.3 A 11.3 C 20.0 Comp. Ex. 12 A 23.3 A 11.3 D25.0

TABLE 7 Measurement Results of Pores Mercury Porosimetry Total PoreMedian Pore Pore Volume of Volume Porosity Diameter A Pore Volume ofMore Than 50 μm⁽²⁾ No. (cm³/g) (%) (μm) 15-40 μm⁽¹⁾ (%) (%) Example 10.588 59.7 26.6 70.3 11.6 Example 2 0.685 63.3 33.0 68.4 12.1 Example 30.460 53.7 25.0 70.8 11.4 Example 4 0.599 60.2 26.1 68.7 13.5 Example 50.561 58.6 25.4 62.4 17.5 Comp. Ex. 1 0.698 63.8 34.0 60.0 21.2 Comp.Ex. 2 0.413 51.0 23.0 71.7 10.2 Example 6 0.550 58.1 25.1 60.7 22.1Example 7 0.598 60.1 26.1 72.0 12.1 Example 8 0.608 60.5 25.5 70.7 13.2Example 9 0.597 60.1 26.7 68.7 12.2 Comp. Ex. 3 0.595 60.0 26.3 72.012.0 Comp. Ex. 4 0.580 59.4 26.5 71.9 13.1 Comp. Ex. 5 0.611 60.6 25.969.1 12.5 Example 10 0.611 60.6 26.9 63.7 18.0 Example 11 0.580 59.426.3 78.1 10.6 Example 12 0.607 60.5 26.8 69.5 11.8 Comp. Ex. 6 0.61060.6 26.9 58.4 21.0 Comp. Ex. 7 0.577 59.3 25.6 82.8 9.2 Comp. Ex. 80.601 60.2 26.7 69.7 11.5 Example 13 0.605 60.4 26.3 67.9 13.3 Example14 0.591 59.8 25.7 73.0 10.3 Comp. Ex. 9 0.612 60.7 26.8 64.9 16.3 Comp.Ex. 10 0.570 59.0 25.5 80.2 9.2 Comp. Ex. 11 0.601 60.2 26.5 65.5 16.5Example 15 1.132 74.0 34.0 60.7 19.7 Example 16 0.480 54.7 25.0 78.310.4 Comp. Ex. 12 1.240 75.8 38.0 50.2 25.4 Note: ⁽¹⁾A ratio of thevolume of pores having diameters of 15-40 μm to the total pore volume.⁽²⁾A ratio of the volume of pores having diameters of more than 50 μm tothe total pore volume. Measurement results of Pores Bubble Point MethodMedian Pore Maximum Pore (A − B)/B × Diameter B Diameter 100 No. (μm)(μm) (%) Example 1 17.8 45.0 49.5 Example 2 20.3 79.0 62.6 Example 316.5 41.0 51.5 Example 4 17.0 50.1 53.5 Example 5 16.0 39.0 58.8 Comp.Ex. 1 25.5 104.0 33.3 Comp. Ex. 2 13.0 32.0 76.9 Example 6 15.0 30.067.3 Example 7 19.3 85.0 35.2 Example 8 15.1 39.0 68.9 Example 9 18.149.0 47.5 Comp. Ex. 3 20.1 101.0 30.8 Comp. Ex. 4 14.9 31.0 77.9 Comp.Ex. 5 18.0 48.5 43.9 Example 10 19.8 96.0 35.9 Example 11 16.1 39.0 63.4Example 12 18.0 46.0 48.9 Comp. Ex. 6 21.9 109.0 22.8 Comp. Ex. 7 13.527.0 89.6 Comp. Ex. 8 18.1 45.8 47.5 Example 13 19.2 89.0 37.0 Example14 15.2 36.0 69.1 Comp. Ex. 9 21.8 108.0 22.9 Comp. Ex. 10 13.4 26.690.3 Comp. Ex. 11 21.7 108.0 22.1 Example 15 20.2 92.0 68.3 Example 1618.0 66.0 38.9 Comp. Ex. 12 21.0 112.0 81.0 Crystals Bulk DensityBending Young's Cordierite Spinel Cristobalite of Filter TEC⁽¹⁾ Strengthmodulus No. (%) (%) (%) (g/cm³) (×10⁻⁷/° C.) (MPa) (Gpa) Example 1 914.0 4.5 0.35 7.1 2.9 1.1 Example 2 91 4.0 4.5 0.32 7.2 1.2 1.0 Example 391 4.0 4.5 0.41 7.1 2.5 1.1 Example 4 91 4.0 4.5 0.35 7.0 2.8 1.2Example 5 91 4.0 4.5 0.36 7.2 1.8 1.0 Comp. Ex. 1 91 4.0 4.5 0.32 7.20.8 1.0 Comp. Ex. 2 90 3.5 4.5 0.43 7.1 2.5 1.1 Example 6 91 3.5 4.50.37 7.0 2.3 1.0 Example 7 91 4.0 4.5 0.35 7.0 2.6 0.9 Example 8 91 4.04.5 0.35 7.0 2.8 1.0 Example 9 91 4.0 4.5 0.35 12.0 2.6 1.0 Comp. Ex. 391 4.0 4.5 0.35 7.1 2.1 1.0 Comp. Ex. 4 91 4.0 4.5 0.36 7.1 2.1 1.0Comp. Ex. 5 91 4.0 4.5 0.35 14.0 2.5 1.1 Example 10 91 4.0 4.5 0.35 7.12.5 0.8 Example 11 91 4.0 5.0 0.36 7.1 2.7 0.9 Example 12 91 4.0 5.00.35 12.5 2.5 1.0 Comp. Ex. 6 91 4.0 5.0 0.35 7.2 1.8 0.9 Comp. Ex. 7 914.0 5.0 0.36 7.1 2.5 1.1 Comp. Ex. 8 91 4.0 5.0 0.35 15.1 2.5 1.0Example 13 91 4.0 5.0 0.35 11.4 2.4 0.9 Example 14 91 4.0 5.0 0.35 5.62.5 0.8 Comp. Ex. 9 91 4.0 5.0 0.35 14.8 1.8 0.9 Comp. Ex. 10 91 4.0 5.00.36 3.5 2.5 1.1 Comp. Ex. 11 91 4.0 5.0 0.35 14.6 1.9 0.9 Example 15 914.0 5.0 0.23 7.2 1.5 0.8 Example 16 91 4.0 5.0 0.40 7.1 2.0 0.9 Comp.Ex. 12 91 4.0 5.0 0.21 7.0 0.9 0.9 Note: ⁽¹⁾A thermal expansioncoefficient between 20° C. and 800° C. Evaluation Results Pressure LossWhen 2 g/L Permeability of Soot Was Capturing No. (×10⁻¹² m²) Captured(kPa) Efficiency Example 1 4.5 Excellent 1.2 Excellent 98.5 ExcellentExample 2 7.5 Excellent 0.1 Excellent 95.8 Good Example 3 2.4 Good 1.4Good 98.6 Excellent Example 4 4.9 Excellent 1.1 Excellent 98.9 ExcellentExample 5 2.2 Good 1.5 Good 98.0 Excellent Comp. Ex. 1 8.5 Good 0.1Excellent 93.0 Poor Comp. Ex. 2 1.1 Poor 1.7 Poor 98.6 Excellent Example6 2.0 Good 1.5 Good 98.8 Excellent Example 7 5.1 Excellent 1.2 Excellent95.5 Good Example 8 2.5 Good 1.4 Good 99.0 Excellent Example 9 4.6Excellent 1.2 Excellent 98.2 Excellent Comp. Ex. 3 5.5 Excellent 1.2Excellent 93.0 Poor Comp. Ex. 4 1.8 Poor 1.6 Poor 98.8 Excellent Comp.Ex. 5 4.6 Excellent 1.2 Excellent 98.2 Excellent Example 10 6.0Excellent 1.2 Excellent 95.7 Good Example 11 2.3 Good 1.5 Good 98.9Excellent Example 12 4.4 Excellent 1.2 Excellent 98.5 Excellent Comp.Ex. 6 5.6 Excellent 1.1 Excellent 91.0 Poor Comp. Ex. 7 1.5 Poor 1.6Poor 98.7 Excellent Comp. Ex. 8 4.5 Excellent 1.2 Excellent 98.6Excellent Example 13 5.9 Excellent 1.2 Excellent 95.4 Good Example 142.4 Good 1.4 Good 98.8 Excellent Comp. Ex. 9 5.7 Excellent 1.1 Excellent90.7 Poor Comp. Ex. 10 1.4 Poor 1.6 Poor 98.6 Excellent Comp. Ex. 11 5.8Excellent 1.2 Excellent 90.5 Poor Example 15 7.5 Excellent 1.0 Excellent95.0 Good Example 16 2.4 Good 1.4 Good 99.1 Excellent Comp. Ex. 12 8.0Excellent 0.8 Excellent 85.0 Poor

It is clear from Table 7 that the ceramic honeycomb filters of Examples1-16 within the present invention had improved efficiency of capturingPM, particularly nano-sized PM, with low pressure loss.

On the other hand, the ceramic honeycomb filter of Comparative Example 1using silica F having a median diameter of more than 60 μm had extremelylow PM-capturing efficiency, with the maximum pore diameter determinedby the bubble point method being more than 100 μm, and the value of[(A−B)/B×100], wherein A represents the median pore diameter of cellwalls measured by mercury porosimetry, and B represents the median porediameter of cell walls measured by the bubble point method, being 35% orless. The ceramic honeycomb filter of Comparative Example 2 using silicaG having a median diameter of less than 15 μm had low permeability andlarge soot-capturing pressure loss, with the value of [(A−B)/B×100]being more than 70%.

The ceramic honeycomb filter of Comparative Example 3 using kaolin Ehaving a median diameter of more than 8 μm had extremely lowPM-capturing efficiency, with the maximum pore diameter determined bythe bubble point method being more than 100 μm, and the value of[(A−B)/B×100] being 35% or less. The ceramic honeycomb filter ofComparative Example 4 using kaolin F having a median diameter of lessthan 1 μM had low permeability and large soot-capturing pressure loss,with the value of [(A−B)/B×100] being more than 70%. The ceramichoneycomb filter of Comparative Example 5 using kaolin G having acleavage index of less than 0.9 had a thermal expansion coefficient ofmore than 13×10⁻⁷/° C. between 20° C. and 800° C.

The ceramic honeycomb filter of Comparative Example 6 using talc Ehaving a median diameter of more than 25 μm had extremely lowPM-capturing efficiency, with the maximum pore diameter determined bythe bubble point method being more than 100 μm, and the value of[(A−B)/B×100] being 35% or less. The ceramic honeycomb filter ofComparative Example 7 using talc F having a median diameter of less than10 μm had low permeability and large soot-capturing pressure loss, withthe value of [(A−B)/B×100] being more than 70%. The ceramic honeycombfilter of Comparative Example 8 using talc G having a morphology indexof less than 0.77 had a thermal expansion coefficient of more than13×10⁻⁷/° C. between 20° C. and 800° C.

The ceramic honeycomb filter of Comparative Example 9 using alumina Dhaving a median diameter of more than 6 μm had extremely lowPM-capturing efficiency, and a thermal expansion coefficient of morethan 13×10⁻⁷/° C. between 20° C. and 800° C., with the maximum porediameter determined by the bubble point method being more than 100 μm,and the value of [(A−B)/B×100] being 35% or less. The ceramic honeycombfilter of Comparative Example 10 using alumina E having a mediandiameter of less than 1 μm had low permeability and large soot-capturingpressure loss, with the value of [(A−B)/B×100] exceeding 70%. Theceramic honeycomb filter of Comparative Example 11 using aluminumhydroxide B having a median diameter of more than 6 μm had extremely lowPM-capturing efficiency and a thermal expansion coefficient of 13×10⁻⁷/°C. between 20° C. and 800° C., with the maximum pore diameter determinedby the bubble point method being more than 100 μm, and the value of[(A−B)/B×100] being 35% or less.

The ceramic honeycomb filter of Comparative Example 12 using thepore-forming material D having a median diameter of more than 70 μm hadextremely low PM-capturing efficiency, with the maximum pore diameterdetermined by the bubble point method being more than 100 μm, and thevalue of [(A−B)/B×100] being more than 70%.

EFFECT OF THE INVENTION

Because the ceramic honeycomb filters of the present invention have lowpressure loss, as well as improved efficiency of capturing PM,particularly nano-sized PM having large influence on humans, they aresuitable as exhaust gas filters for diesel engines.

What is claimed is:
 1. A method for producing a ceramic honeycomb filtercomprising the steps of preparing a cordierite-forming materialcontaining talc, silica, an alumina source and kaolin; classifying thecordierite-forming material by passing the cordierite-forming materialthrough a sieve having opening diameters of 250 μm or less to form aclassified cordierite-forming material; blending the classifiedcordierite-forming material and a pore-forming material to prepare amoldable material; extruding said moldable material to form ahoneycomb-shaped molding; and plugging the predetermined flow paths ofsaid honeycomb-shaped molding to form said ceramic honeycomb filter;said silica having a median diameter of 15-58 μm, said talc having amedian diameter of 10-25 μm and a morphology index of 0.77-0.84, saidkaolin having a median diameter of 1.5-7.5 μm and a cleavage index of0.9-0.95, said cleavage index being a value expressed byI₍₀₀₂₎/[I₍₂₀₀₎+I₍₀₂₀₎+I₍₀₀₂₎], wherein I₍₂₀₀₎, I₍₀₂₀₎ and I₍₀₀₂₎ are thepeak intensities of (200), (020) and (002) planes measured by X-raydiffraction, said alumina source having a median diameter of 1.5-6 μm,and said pore-forming material comprising pore-forming materialparticles having a median diameter of 30-70 μm.
 2. A method forproducing a ceramic honeycomb filter comprising the steps of preparing acordierite-forming material containing talc, silica, an alumina sourceand kaolin; classifying the cordierite-forming material by passing thecordierite-forming material through a sieve having opening diameters of250 μm or less to form a classified cordierite-forming material;blending the classified cordierite-forming material and a pore-formingmaterial to prepare a moldable material; extruding said moldablematerial to form a honeycomb-shaped molding; and plugging thepredetermined flow paths of said honeycomb-shaped molding to form saidceramic honeycomb filter; said silica having a median diameter of 15-58μm, said talc having a median diameter of 10-25 μm and a morphologyindex of 0.77-0.84, said kaolin having a median diameter of 1.5-7.5 μmand a cleavage index of 0.9-0.95, said cleavage index being a valueexpressed by I₍₀₀₂₎/[I₍₂₀₀₎+I₍₀₂₀₎+I₍₀₀₂₎], wherein I₍₂₀₀₎, I₍₀₂₀₎ andI₍₀₀₂₎ are the peak intensities of (200), (020) and (002) planesmeasured by X-ray diffraction, said alumina source having a mediandiameter of 1.5-6 μm, and said pore-forming material comprisingpore-forming material particles having a median diameter of 30-70 μm,wherein in a curve showing the relation between a particle diameter ofsaid pore-forming material particles and a cumulative volume of saidpore-forming material particles, a particle diameter d90 of saidpore-forming material particles at a cumulative volume corresponding to90% of a total volume of the pore-forming material particles is 50-90μm.
 3. The method for producing a ceramic honeycomb filter according toclaim 1, wherein said alumina source has a median diameter of 2-5 μm. 4.The method for producing a ceramic honeycomb filter according to claim1, wherein said silica has a median diameter of 35-55 μm.
 5. The methodfor producing a ceramic honeycomb filter according to claim 2, whereinsaid alumina source has a median diameter of 2-5 μm.
 6. The method forproducing a ceramic honeycomb filter according to claim 2, wherein saidsilica has a median diameter of 35-55 μm.
 7. The method for producing aceramic honeycomb filter according to claim 3, wherein said silica has amedian diameter of 35-55 μm.
 8. The method for producing a ceramichoneycomb filter according to claim 5, wherein said silica has a mediandiameter of 35-55 μm.
 9. A method for producing a ceramic honeycombfilter comprising the steps of preparing a cordierite-forming materialcontaining talc, silica, an alumina source and kaolin; classifying thecordierite-forming material by passing the cordierite-forming materialthrough a sieve having opening diameters of 250 μm or less to form aclassified cordierite-forming material; blending the classifiedcordierite-forming material and a pore-forming material to prepare amoldable material; extruding said moldable material to form ahoneycomb-shaped molding; and plugging the predetermined flow paths ofsaid honeycomb-shaped molding to form said ceramic honeycomb filter;said silica having a median diameter of 15-58 μm, said talc having amedian diameter of 10-25 μm and a morphology index of 0.77-0.84, saidkaolin having a median diameter of 1.5-7.5 μm and a cleavage index of0.9-0.95, said cleavage index being a value expressed byI₍₀₀₂₎/[I₍₂₀₀₎+I₍₀₂₀₎+I₍₀₀₂₎], wherein I₍₂₀₀₎, I₍₀₂₀₎ and I₍₀₀₂₎ are thepeak intensities of (200), (020) and (002) planes measured by-X raydiffraction, said alumina source having a median diameter of 1.5-6 μm,and said pore-forming material comprising pore-forming materialparticles having a median diameter of 30-70 μm, wherein in a curvehaving the relation between a particle diameter of said pore-formingmaterial particles and a cumulative volume of said pore-forming materialparticles, a particle diameter d90 of said pore-forming materialparticles at a cumulative volume corresponding to 90% of a total volumeof the pore-forming material particles is 60-80 μm.
 10. The method forproducing a ceramic honeycomb filter according to claim 9, wherein saidalumina source has a median diameter of 2-5 μm.
 11. The method forproducing a ceramic honeycomb filter according to claim 9, wherein saidsilica has a median diameter of 35-55 μm.
 12. The method for producing aceramic honeycomb filter according to claim 10, wherein said silica hasa median diameter of 35-55 μm.